This invention relates to a method of manufacturing carbon electrodes, and more particularly to a method of manufacturing carbon electrodes which enables the elimination of unevenness of quality between each lot of products, i.e. carbon electrodes, to improve the quality thereof and provide carbon electrodes which have a reduced basic consumption unit and are free from abnormal breakages.
Carbon material is generally manufactured in accordance with the steps shown in FIG. 1. A raw material, for example petroleum coke or regular coke 1, is broken up into comparatively large particles in a coarse grinder 2, the resultant coke is sorted in a sieving unit 3 and is then broken up into fine particles if necessary in a pulverizer 4. The distribution of sizes of the pulverized coke is adjusted, and the different sizes are then stored in storage vessels 5.
In the meantime, a binder consisting, for example, of coal tar pitch is placed in a storage vessel 6. The grain size-adjusted coke and binder are kneaded in a kneader 7 while the materials are heated. The kneaded product is then formed by extrusion molding or cast molding.
To extrusion-mold the kneaded mixture, it is cooled in a cooler 8 and then extruded through a nozzle of a predetermined shape by an extruder 9.
To cast-mold the kneaded mixture, it is cooled to room temperature and then re-pulverized in a re-pulverizer 10 to re-adjust the grain size distribution thereof. The re-pulverized mixture is compression-molded in a molding machine 11.
The molded product 12 thus obtained is baked in a baking furnace 13 and then impregnated with pitch in an impregnator 14. The resultant mass is re-baked to obtain a baked product 15, which is heat-treated if necessary with graphitized lead 16 to obtain a graphitized product 17.
To manufacture carbon electrodes by this method, the kneaded starting material is extruded to a predetermined length to obtain molded products for the carbon electrodes.
However, in such a conventional method of manufacturing carbon electrodes, it is difficult to remove bubbles and water, which have been entrained in the kneaded product, while the kneaded product is being compressed in a container 20. Since these bubbles and water act as cushions during the compressions, unevennesses in the degree of compression occur in the molded product. Accordingly, the molded products for carbon electrodes extruded from a nozzle 25 inevitably have parts of different densities.
Therefore, a carbon electrode obtained by baking such a molded product has unevennesses in density, as mentioned above, so that it does not expand uniformly when heated. For example, when an arc discharge is formed between such a carbon electrode and molten steel in a steel-making arc furnace, the carbon electrode chips or breaks due to its uneven thermal expansion, so that a stable arc discharge cannot be maintained over a long period of time. Whenever such a carbon electrode is damaged, it must be re-adjusted or replaced.
Furthermore, the quality of the molded and baked carbon electrode also varies with variations in the quality of the raw coke and binder, so that it is difficult to manufacture carbon electrodes of constant quality.
Carbon electrodes usually have the properties shown in Table 1.
TABLE 1 ______________________________________ High-load electrode Nipple ______________________________________ Bulk density (g/cc) 1.65-1.70 1.75-1.80 Specific electrical 5.0-6.5 4.0-5.0 resistance (.mu..OMEGA.m) Bending strength 100-130 220-300 (Kgf/cm.sup.2) Young's modulus 800-1000 1200-1500 (Kgf/mm.sup.2) Coefficient of thermal 1.0-1.2 0.9-1.1 expansion (.times. 10.sup.-6 /.degree.C.) ______________________________________
A high-load electrode is an electrode used to improve the productivity of a steel-making arc furnace by applying thereto during a short period of time ultra-high power in much higher than the electrical power applied to an electrode in an electric furnace. Nipples are joints connecting a plurality of carbon electrodes together so that the electrodes can be used in a line in an arc furnace. The nipples have a diameter smaller than that of the carbon electrodes, and receive a high mechanical load when used as joints. The nipples through which a large current is to flow are provided with taper screws at both end portions thereof, and are made of the same carbon material as the carbon electrodes.
A carbon electrode should preferably have a high bulk density, a low specific electrical resistance, a high bending strength, a comparatively low Young's modulus, and a low coefficient of thermal expansion.
These five properties of a carbon electrode are mutually related; specific electrical resistance decreases in inverse proportion to bulk density and the bending strength, Young's modulus and coefficient of thermal expansion increase in proportion thereto.
However, a carbon electrode having a high Young's modulus and a high coefficient of thermal expansion is more likely to break or chip while in use; such a carbon electrode is undesirable.
Therefore it is necessary to increase the bulk density of the carbon electrode without producing an increase in the Young's modulus and coefficient of thermal expansion thereof.
Two kinds of raw coke are used in the production of carbon electrodes. They are pitch coke obtained from coal, and coke obtained from petroleum. The properties of these two kinds of coke are compared in Table 2.
TABLE 2 ______________________________________ Coefficient of Bulk density thermal expansion ______________________________________ Pitch coke 1.64-1.66 0.7-0.75 Petroleum coke 1.67-1.70 0.9-1.0 ______________________________________
When petroleum coke is used as the raw material for manufacturing carbon electrodes, both the bulk density and the coefficient of thermal expansion of the final products increase.
When pitch coke is used for manufacturing carbon electrodes, both the bulk density and coefficient of thermal expansion of the final products decrease.
Therefore, if a method enabling the production of a carbon electrode with an increased bulk density from pitch coke is discovered, desirable effects can be obtained.